3.3.2 Gas Exchange

Question 1

explain why stomata open due to increase in light intensity (1)

Answer 1

allowing carbon dioxide to enter for photosynthesis;
Or
for gas exchange allowing photosynthesis

Question 2

Describe how carbon dioxide in the air outside a leaf
reaches mesophyll cells inside the leaf (4)

Answer 2

1. (Carbon dioxide enters) via stomata; Reject stroma
2. (Stomata opened by) guard cells;
3. Diffuses through air spaces;
4. Down diffusion gradient;

Question 3

describe & explain an advantage and disadvantage to having a higher stomatal density

Answer 3

Advantage
1. More carbon dioxide uptake;
2. More photosynthesis so faster/more growth;
Disadvantage
3. More water loss/transpiration
Accept plant wilts for ‘more water loss’
4. Less photosynthesis so slower/less growth;

Question 4

Describe and explain Adaptations to desert plants (6)

Answer 4

1. Hairs so ‘trap’ water vapour and water potential
   gradient decreased;
2. Stomata in pits/grooves so ‘trap’ water vapour and
   water potential gradient decreased;
3. Thick (cuticle/waxy) layer so increases diffusion
   distance;
4. Waxy layer/cuticle so reduces
   evaporation/transpiration;
5. Rolled/folded/curled leaves so ‘trap’ water vapour and
   water potential gradient decreased;
6. Spines/needles so reduces surface area to volume
   ratio;

Question 5

Describe the counter-current mechanism (3)

Answer 5

1. Water and blood flow in opposite directions;
2. Blood always passing water with a higher oxygen
   concentration;
3. Diffusion/concentration gradient (maintained) along
   (length of) lamella/filament;

Question 6

Explain two ways in which the structure of fish gills is adapted for efficient gas exchange.(2)

Answer 6

1. Many lamellae / filaments so large surface area;
2. Thin (surface) so short diffusion pathway;

Question 7

Describe & explain how the structure of the insect gas exchange system:
• provides cells with sufficient oxygen

Answer 7

1. Spiracles (lead) to tracheae (that lead) to
   tracheoles;
2. Open spiracles allow diffusion of oxygen from air
   OR
   Oxygen diffusion through tracheae/tracheoles;
3. Tracheoles are highly branched so large surface
   area (for exchange);
4. Tracheole (walls) thin so short diffusion distance
   (to cells)
   OR
   Highly branched tracheoles so short diffusion distance
   (to cells)
   OR
   Tracheoles push into cells so short diffusion distance;
5. Tracheole walls are permeable to oxygen;

Question 8

Describe & explain how the structure of the insect gas exchange system: limits water loss.(2)

Answer 8

1. Cuticle/chitin in tracheae impermeable so reduce
   water loss;
2. Spiracles close (eg.during inactivity) preventing
   water loss;

Question 9

Describe and explain Abdominal Pumping (3)

Answer 9

1. Abdominal pumping/pressure in tubes linked to
   carbon dioxide release;
2. (Abdominal) pumping raises pressure in body;
3. Air/carbon dioxide pushed out of body /air/carbon
   dioxide moves down pressure gradient (to atmosphere)

Question 10

Explain three ways in which an insect’s tracheal system is adapted for efficient gas exchange.

Answer 10

1. Tracheoles have thin walls so short diffusion
   distance to cells;
2. Highly branched / large number of
   tracheoles so short diffusion distance to cells;
3. Highly branched / large number of
   tracheoles so large surface area (for gas exchange);
4. Tracheae provide tubes full of air so fast diffusion
   (into insect tissues);
5. Fluid in the end of the tracheoles that moves out
   (into tissues) during exercise so faster diffusion through
   the air to the gas exchange surface;
   OR
   Fluid in the end of the tracheoles that moves out (into
   tissues) during exercise so larger surface area (for gas
   exchange);
6. Body can be moved (by muscles) to move
   air so maintains diffusion / concentration gradient for
   oxygen / carbon dioxide;

Question 11

Describe and explain one feature of the alveolar
epithelium that makes the epithelium well adapted as a surface for gas exchange.

Answer 11

Mark in pairs
1. Flattened cells
OR
Single layer of cells;
Reject thin cell wall/membrane Accept thin cells
Accept ‘one cell thick’
2. Reduces diffusion distance/pathway;
3. Permeable;
4. Allows diffusion of oxygen/carbon dioxide;
5. moist
6. Increase rate of diffusion

Question 12

describe and explain inhaling (4)

Answer 12

1. Diaphragm (muscle) contracts and external
   intercostal muscles contract;
   Ignore ribs move up and out
2. (Causes volume increase and) pressure decrease;
3. Air moves down a pressure gradient
   Ignore along
   OR
   Air enters from higher atmospheric pressure;

Question 13

Describe the pathway taken by an oxygen molecule from an alveolus to the blood. (2)

Answer 13

1. (Across) alveolar epithelium;
2. Endothelium of capillary;

Question 14

Explain how one feature of an alveolus allows efficient gas exchange to occur

Answer 14

1. (The alveolar epithelium) is one cell thick;
   Reject thin membrane
2. Creating a short diffusion pathway / reduces the
   diffusion distance;

Question 15

Describe the gross structure of the human gas exchange system
(1)

Answer 15

1. Named structures – trachea, bronchi, bronchioles,
   alveoli;

Question 16

Describe how we breathe in and out.(4)

Answer 16

1. Breathing in – diaphragm
   contracts and external intercostal muscles contract;
2. (Causes) volume increase and pressure decrease
   in thoracic cavity (to below atmospheric, resulting in air
   moving in);
   For thoracic cavity accept ‘lungs’ or ‘thorax’.
   Reference to ‘thoracic cavity’ only required once.
3. Breathing out - Diaphragm
   relaxes and internal intercostal muscles contract;
   Accept diaphragm relaxes and (external) intercostal
   muscles relax and lung tissue elastic (so recoils).
4. (Causes) volume decrease and pressure increase
   in thoracic cavity (to above atmospheric, resulting in air
   moving out);